From 1996 to 2001, Rehberg managed the Rehberg Ranch near Billings. He oversaw a herd of 500 cattle and 600 cashmere goats.[6] After being elected to congress, Rehberg gave up ranching, citing the difficulty of managing a herd whilst travelling between Montana and Washington D.C.[7][8]

In 1977 he began working as an intern in the Montana State Senate, and two years later he joined the Washington, D.C. staff of Montana U.S. Congressman Ron Marlenee as a legislative assistant. In 1982, Rehberg returned to farming, until running for the State House in 1984.

Rehberg was elected then to the Montana State House of Representatives from 1985 to 1991, where he served three terms. In the legislature, he considered himself to be a fiscal conservative, and he advocated balancing the state budget without any tax increases. He was the only freshman member to serve on the House Appropriations Committee.

In July 1991, Rehberg was appointed Lieutenant Governor by Governor Stan Stephens. He was subsequently elected to a full term in 1992.

As Lieutenant Governor, Rehberg sought to bring government back to the local level by traveling to all 56 counties every year. He chaired the Drought Advisory Committee and the Task Force credited with reforming Worker’s Compensation, the Montana Rural Development Council, and several health care initiatives. He was succeeded as Lieutenant Governor by fellow Republican Judy Martz.

In 2012 Rehberg co-sponsored "H.R. 1505"(PDF). Retrieved 2012-08-14. which would waive environmental laws that would otherwise prevent the Department of Homeland Security from constructing roads, fences, and forward operating bases in national parks and wilderness areas within 100 miles of the international border. Homeland Security would not have to inform affected parties before pursuing these activities.[17] The bill's dissenters claim that it "employs a manufactured conflict with border security to weaken [environmental laws]." The Department of Homeland Security called the bill "unnecessary and bad policy" since DHS already has a memorandum of understanding allowing them to enter these lands without prior approval.[18]

After having scrutinized the Pell Grant program as one that was 'expanding' too quickly, congressman Rehberg set his sights on the free school lunch program as a program where there was potential of taxpayers being ripped off, although the state Superintendent of Public Instruction Denise Juneau expressed the real concern that not enough families that qualify request the assistance.[20]

Rehberg served on the Military Quality of Life and Foreign Operations Subcommittees of the powerful House Committee on Appropriations. In 2007, he voted against the Mandatory Troop Rest Periods between Deployments to Iraq bill (creates a mandatory rest period between deployments to Iraq for members of the Armed Forces, passed) and against the Redeployment from Iraq Act (reduces the presence of the U.S. armed forces in Iraq by April 1, 2008, passed).[23]

Environmental record

Rehberg describes himself as “a lifelong steward of the land”. He voted against the Renewable Energy and Energy Conservation Tax Act of 2007 and against the Securing America’s Future Energy (SAFE) Act of 2001.[24] He is currently one of the foremost critics on keeping the gray wolf on the endangered species list.[25] On environmental issues, Rehberg has also been given low ratings by interest groups. Environment America gave him an 8% rating in 2008, and 27% rating in 2009. Defenders of Wildlife Action Fund gave him a 13% rating. In his position statement on Economic Development, he said “Our [Montana’s] coal and natural gas reserves stretch for hundreds of years into the future, and we can turn them into much needed energy.” Dennis Rehberg states opposition to the Shays-Meehan bill from 1998, of which he says it “stripped freedom of speech rights from Montana citizens.” Aside from the bill, he makes a point to say he supports full and open disclosure laws for campaign finance reform.

LGBT issues

Rehberg opposes same-sex marriage, and supports a constitutional amendment defining marriage as between one man and one women.[26] He voted against the Local Law Enforcement Hate Crimes Prevention Act of 2007, a bill that amends the Federal definition of a hate crime to include gender identity and sexual orientation. The bill passed, 237 to 180.[27] In the Dec. 26, 1994, issue of Newsweek, p. 83, then Lt. Governor Rehberg was quoted as saying "The problem with AIDS is: you got it, you die. So why are we spending money on the issue?," when discussing cuts to the state's hospital budget. [28]

On February 6, 2011, Rehberg announced that he would challenge Senator Jon Tester (D-MT) in 2012.[30] The outcome of the race was expected to have a significant impact on which party controls the United States Senate during the 113th Congress.[31]

According to Salon, it was a "race that pundits are saying could be a tough challenge for Tester."[32] Political Scientist Larry Sabato predicted a narrow Rehberg victory, pointing out that Rehberg had led in 10 of 13 recent polls.[33] However, Tester eventually defeated Rehberg, 48.7%–44.8%.[34]

When after one term in the U.S. House Steve Daines ran for the U.S. Senate, Rehberg considered running for his old House seat, but ultimately declined.[35] The seat was ultimately won by Republican Ryan Zinke.

With a net worth of between $6.5 and $54 million, Rehberg was the fourteenth-richest U.S. Representative in the House in 2008.[36] However, in an April 2011 town hall meeting video released by the Montana Democratic Party, he told an audience member that he was "land-rich and cash-poor" and "struggling like everyone else."[37]

On the night of Thursday, August 27, 2009, while in the area for a series of town hall forums on healthcare, Rehberg was a passenger in a boating accident near Bigfork, MT on Flathead Lake. Montana State Senate Majority Whip Greg Barkus was driving the boat.[32][38]

Rehberg, Barkus and his wife Kathy, and two Rehberg aides, Dustin Frost and Kristin Smith, were hospitalized in Kalispell following the accident, which took place sometime between 10pm and midnight. Frost, Rehberg's then 27-year-old state director in Montana, suffered a brain injury. Frost was in a coma for more than a week.[38] Rehberg sustained a broken ankle and rib fractures.[39]

Law enforcement agents investigated the cause of the accident, including "how fast the boat was going, who was driving, and 'whether alcohol and drugs were involved.'" [38] Barkus "was found to have a blood alcohol content of .16 two hours after the accident. Despite the fact that, according to a witness quoted in the criminal complaint, Barkus had been drinking scotch and wine at a lakeside restaurant that night, Rehberg later said he 'was surprised to learn the results of Greg's blood alcohol test'."[32] Rehberg released his BAC, measured several hours after the crash, at an alleged .05 percent.[40]

Barkus, ultimately pleaded no contest to a felony criminal endangerment charge. He was given a four-year deferred sentence, probation, paid $4,000 in restitution and a $25,000 fine.[41]

In July 2010, Rehberg's corporation that has developed a subdivision on his former ranch land sued the City of Billings for calling back firefighters from protecting trees and some scrub brush. The City of Billings had recently annexed a significant amount of undeveloped grass-land, including Rehberg's ranch, and had not developed a firefighting policy for wildfires, which are significantly different from structure fires. When the fire flared again, the city firefighters had to return to put out the fire after significant damage was done.[42] While the suit was filed in July, Rehberg and his wife did not push forward with litigation, given the political pressures of the 2010 Congressional campaign.[43]

The city of Billings spent nearly $21,000 defending itself against the lawsuit before it was dropped by Rehberg.[44]